Horton

Re: Removing unneeded self joins

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: jian he <jian.universality@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Andrei Lepikhov <a.lepikhov@postgrespro.ru>, Richard Guo <guofenglinux@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-07-04T03:04:19Z
Lists: pgsql-hackers

Commits

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  1. Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

  2. Put enable_self_join_elimination into postgresql.conf.sample

  3. Get rid of ojrelid local variable in remove_rel_from_query()

  4. Implement Self-Join Elimination

  5. Revert: Remove useless self-joins

  6. Replace lateral references to removed rels in subqueries

  7. Replace relids in lateral subquery parse tree during SJE

  8. Forbid SJE with result relation

  9. Fix misuse of RelOptInfo.unique_for_rels cache by SJE

  10. Replace the relid in some missing fields during SJE

  11. Revert 56-bit relfilenode change and follow-up commits.

  12. Stabilize timetz test across DST transitions.

  13. Speed up finding EquivalenceClasses for a given set of rels

  14. Fix mark-and-restore-skipping test case to not be a self-join.

On Thu, Jul 4, 2024 at 5:15 AM jian he <jian.universality@gmail.com> wrote:
> in remove_self_join_rel, i have
> ```ChangeVarNodes((Node *) root->parse, toRemove->relid, toKeep->relid, 0);```
> which will change the joinlist(RangeTblRef)  from (1,2)  to (2,2).
> Immediately after this call, I wrote a function (restore_rangetblref)
> to restore the joinlist as original (1,2).
> then remove_rel_from_joinlist won't error out.
> see remove_self_join_rel, restore_rangetblref.

Thank you, now this is clear.  Could we add additional parameters to
ChangeVarNodes() instead of adding a new function which reverts part
of changes.

> current mechanism, in this example context,
> SJE can translate ```update test set val = t.val + 1 from test t where
> test.id = t.id;``` as good as to
> ```update test set val = val + 1```.
> if we replace it that way, then this example would result val = 3.
>
> but without SJE,
> ```update test set val = t.val + 1 from test t where test.id = t.id;```
> will result val = 2.
>
> you mentioned the EPQ problem,  previously i don't know what that means.

Yes, I guessed so.  I should have come with more detailed explanation.

> now i see, I feel like it is quite challenging to resolve it.

Yep.  Glad to see we are on the same page.

This is why I think we could leave SJE for target relation of
modification queries for future.  I'd like to not devalue SELECT-only
SJE, given that this is a step forward anyway.

------
Regards,
Alexander Korotkov
Supabase